Profile distribution of phosphorus fractions in rice and associated non-rice soils under different pedogenic development stages of the Brahmaputra valley of Assam

Profile distribution of different forms of phosphorus (P) was studied in rice and associated non-rice soils belong to Entisols, Inceptisols and Alfisols of the Brahmaputra Valley, Assam to assess the P availability and evaluate the relationship between P distribution and pedogenic development of the soils. The modified Hedley fractionation method was adopted for the purpose. The overall sequence of abundance for different P fractions of the soils based on averages of profile weighted mean was as: NaOH extracted organic P (167.0 mg kg^-1) > NaOH extracted inorganic P (112.9 mg kg^-1) > residual P (45.2 mg kg^-1) > bicarbonate extracted organic P (33.1 mg kg^-1) > dilute HCl extracted P (24.0 mg kg^-1)> bicarbonate extracted inorganic P (23.7 mg kg^-1) > concentrated HCl extracted organic P (20.7 mg kg^-1) > concentrated HCl extracted inorganic P (18.2 mg kg^-1) > resin P (5.8 mg kg^-1). The organic P extracted by NaOH was emerged as the most dominant fraction in the studied soil and contribute 37.0% to the total sum of the P fractions. The distribution of P was mostly influenced by pedogenic development stages of the soils rather than land use system. The total inorganic P fractions showed an increasing trend from Entisols to Alfisols but total organic P decreased with the maturity of soils. The significant and positive relationship between labile P fractions (resin P, bicarbonate extracted organic and inorganic P) with other organic P fractions indicated a stable and consistent P supplying capacity of the soils.